A method has been devised for the study of rat antral mucosa in perifusion chambers for the determination of the response of gastrin cells to inhibitory and stimulating factors and the ultrastructural appearance of the gastrin cell following incubation. A collaborative study with other investigators to clarify conflicting reports of the light microscopic staining reaction of the antral gastrin cell is in progress. The possibility that gastrin cells are present in cultures of fetal rat pancreas and isolated islets of adult rats is being explored.